1. Field of Invention
This invention relates to digital image processing in general, and to color normalization of images from multiple sources in particular.
2. Description of Related Art
Various methods and systems exist for combining several digital images into an extended image mosaic. This is desirably accomplished in a manner that yields a composite image or mosaic that looks like a single image, for example, without seams or other merging features. Boundaries between neighboring image segments that overlap or nearly overlap often have substantial differences in contrast, intensity, resolution and color which appear as visible seams in the composite image or mosaic.
Multicamera video systems typically need color calibration. Calibrating color balance and shutter speed on all video cameras has a disadvantage of being time consuming, and may sacrifice the dynamic range of the multicamera system. Moreover, in outdoor settings, it is difficult to avoid situations where one camera image region has direct sun and other cameras image regions have various degrees of shade. If all cameras use a uniform shutter speed, then the camera image region having direct sun is likely to be overexposed and the other camera image regions are likely to be underexposed. However, using automatic gain control to compensate for large dynamic scene brightness ranges can lead to objectionable image seams when the gain of neighboring cameras is significantly different.
U.S. Pat. No. 6,075,905, for example, merges color images based on a comparison between colors in the overlap regions between the individual images which form the composite or mosaic image. For two overlapping images, the 905 patent performs a least-square fit over the image-overlap region to determine the color-space affine transformation among the RGB composite signal that adjusts the colors of the second image so that the colors of the second image in the image-overlap region most closely match the colors of the first image-overlap region. The resulting affine transformation is then applied to the entirety of the second image. Extending the objective function to more than two overlapping images is done by ascribing an affine transformation to all but one of the images, the transformations being with respect to the untransformed, or reference, image, and then by adding the squared RGB color differences over all the pixels in all overlap regions.
U.S. Pat. No. 6,349,153, for example, merges color images and uses the information in the overlap region between images to correct the color of the two images. To compensate for unwanted artifacts of color bands at the overlap region, the color of the images is adjusted based on pixel information from the overlap region so as to bring the two images into line with each other. Brightness, contrast and gamma parameters in the overlap region are used to modify image color intensity. The 153 patent tapers the color correction so that full color correction is applied within the overlap region. Outside the overlap region, only a fraction of the correction is applied, where the fraction preferably tapers from 100% to 0% as the pixel distance from the overlap region increases.
U.S. Pat. No. 6,278,463, for example, processes first and second digital images, including color images, with overlapping image content defining an overlap region including common image data. Color image pixel data in the overlap region is processed to produce a composite image.